The present invention relates broadly to an amplifier apparatus, and in particular to a variable gain amplifier apparatus.
There are a number of undesirable conditions that may occur in amplifiers, produced by improper circuit design and by inherent limitations in the physical operation of the devices. In general, good circuit design can reduce all of the undesirable conditions, including those caused by physical limitations to the point where they are not noticeable.
If the output signal from an amplifier is not an exact replica of the input signal, distortion has occurred. In theory it is impossible for an amplifier to avoid introducing distortion. On the other hand, amplifiers have been designed in which the distortion is extremely small. Distortion is introduced by two factors in the amplifier. One factor is a nonlinear relation of the device between the input signal and the output signal. If the input signal is a sinusoidal signal, the output will be composed of harmonies of the input. The second factor is the frequency response of the amplifier. Since the gain of the amplifier is not the same for all frequencies, the amplification of each harmonic component of an input signal is not necessarily the same as that of the others. The output signal, since it is a superposition of the harmonic components, differs from the input signal. Distortion caused by nonlinearities in the amplifier and by the frequency response of the amplifier can be reduced by the proper use of feedback.
The noise encountered in amplifiers may in general, be classified into thermal noise and shot noise. Thermal noise is caused by the random motion of electrons inside resistors, conductors, tubes, and transistors. It has the characteristic of having uniform power per unit bandwidth. Furthermore, the noise power is directly proportional to the temperature when expressed in degrees Kelvin. Shot noise is the name given to the noise generated in transistors or vacuum tubes by the random emission of holes or electrons from the emitter or cathode. The random emission produces minute fluctuations in the average value of collector or plate current. These fluctuations produce a small noise voltage of the same order of magnitude as thermal noise (a few microvolts).
To analyze the effect of these noise sources upon the circuit and to design the optimum circuit, noise generators are used, and the analysis proceeds in a straightforward manner. Although noise voltages can become bothersome in audio amplifiers, the most common situation where they must be considered is the design of rf amplifiers, where quite often the signal to be amplified is not much larger than the rms value of the noise voltage.